Axial flow rotary machines, such as gas turbine engines, have pluralities of rotor blades and stator vanes disposed in serial fashion within the compression section and turbine section of the engine. The blades and vanes direct the flow of working medium gases through the engine. As the gases are passed through the engine, energy is transferred between the rotor blades and the working medium gases. In the compression section, each rotor blade has an airfoil which does work on the working medium gases to compress the gases. The surface of the airfoil is defined by a plurality of airfoil sections disposed about the spanwise axis. In the turbine section, the airfoil receives work from the working medium gases as the gases are expanded. In industrial turbines, the flow is discharged through a free turbine to develop rotational energy. In gas turbine engines for aircraft, the flow is then discharged from the engine to generate useful thrust.
The turbofan engine is one type of gas turbine engine used to power aircraft. The turbofan engine has a large fan having fan blades designed to compress working medium gases entering the jet portion of the engine and working medium gases entering the fan (or by-pass) duct.
It is critical for the aerodynamic efficiency of these engines to form within predetermined limits the contour of the leading edge and the trailing edge of the airfoil. This task is complicated in very large airfoils, such as the airfoil for a fan blade, by the extent of the airfoil which may extend for a length of a meter or more. In addition, the airfoil has an amount of predetermined twist and an edge which varies in thickness by a predetermined amount along its spanwise length.
One approach is described in U.S. Pat. No. 5,055,752 issued to Leistensnider et al. which is assigned to the assignee of the present invention. Leistensnider describes numerically controlling the machining system by probing the surface of the work piece along the length of the edge to determine its exact dimension and location. Leistensnider also describes generating and storing data indicative of dimensions and locations for the final product and machining the edge of the work piece under the direction of the machine program which utilizes that data and other preselected part design data to enable a cutting tool to follow the actual edge of the part.
Another approach to shaping the final contour of the airfoil edge is to perform the shaping as a separate operation after the airfoil has at least been partially formed such that portions of the sides of the airfoil have its final configuration, but the leading edge and trailing edges do not. Variations do occur from airfoil to airfoil in intermediate forms (and in final form) for a given engine even though each of the airfoils falls within the predetermined acceptable limits at these finally formed surfaces. Accordingly, a single machine tool having preprogrammed parameters based on these reference surfaces may or may not generate a leading edge whose final contour meets the stringent requirements for airfoil shape along its length.
This approach is particularly suited to making hollow rotor blades, such as hollow fan blades. This approach partially forms the airfoil in flat form and machines the chordwise dimensions of the airfoil leaving the leading edge and trailing edge of the airfoil with an essentially square-like profile. At the time that the chord dimensions are machined, the surface of the airfoil is machined in the edge region adjacent to the edge at a predetermined angle on one of the surfaces, such as suction surface. The concave side serves as a datum edge which allow hand operations to form the airfoil along its length. Prior to hand forming, the flat, hollow airfoil is placed in a mold under an elevated pressure and temperature for hours to twist the airfoil about a spanwisely extending axis. The airfoil is then placed in a second mold and finally formed at another elevated pressure and temperature. The airfoil is then ready for hand operations on the edge.
The hand operations employ files and template gauges having the thickness and the shape that is required. Site gauges are used by the person performing the hand work to form the final edge. As would be expected, there are minor variations between the edges of airfoils that are formed by the same operator even though the edge does meet blueprint dimensions.
The above art notwithstanding, scientists and engineers working under the direction of Applicants' assignee have sought to develop an apparatus and method for generating an airfoil edge which would decrease the variability of the edge with respect to blueprint dimensions while increasing the speed with which an edge may be formed.